Transport and Retention of Biochar Nanoparticles and Associated Contaminants in Natural Soils

Land application of biomass-derived biochar has been increasingly recommended as a beneficial soil amendment for nutrients (such as N, P) retention. However, the small-scale biochar particles, especially those in the nano-scale range, may migrate or carry nutrients downward the soil profile, reducing nutrition retention and posing a potential risk to the groundwater. In this study, column experiments were conducted to investigate the transport and retention of wood chip-derived biochar nanoparticles (NPs), and its effects on retention and transport of phosphorus (P) in two acidic and two alkaline soils. Higher mobility of biochar NPs was observed in the soil at lower ionic strengths, with CaCl₂ electrolyte being more effective than NaCl in decreasing biochar NPs transport. Humic acid increased the mobility of biochar NPs, likely due to enhanced electrosteric repulsive interactions. In acidic paddy and red soils, biochar NPs facilitated the retention of P, increasing by about 24% and 16%, respectively, compared to the biochar absence. It is because biochar NPs stabilize soil Fe/Al oxides, thereby reducing the release of Fe/Al oxides-associated P. In contrast, in alkaline huangmian and chao soils, retention of P was reduced in the presence of biochar NPs, decreasing by about 23% and 18%, respectively. It was mainly due to the increased transport of Fe/Al oxides-associated P into the effluent. Moreover, biochar NPs could also act as a P carrier, mediating the retention of P. The diffusive gradients in thin films provided in-suit measurement of labile P in soil profiles, showing much lower labile P from retained P in acidic soils than that from alkaline soils though the labile P with biochar NPs presence was increased in all soils. Our findings indicated that the transport of wood chip biochar NPs is significant in the paddy soil, highlighting the contrasting effects on the retention of P in acidic and alkaline soils, implying the cautious land applications of biochar for nutrients retention in soils with different acidities.